Southwestern Corn Borer

W196 Corn Insects Southwestern Corn Borer Scott Stewart, Professor, Entomology and Plant Pathology Angela Thompson McClure, Associate Professor, Plant Sciences and Russ Patrick, Professor, Entomology and Plant Pathology

Classification and initially translucent white or yellowish with black Description: The spots on the body. Older larvae are creamy white southwestern corn and have more distinctive black spots. Larvae reach borer (Diatraea a maximum length of 1¼ inches. Pupae are dark grandiosella, brown, about ¾ inch long and located in the stalk Lepidoptera: or occasionally in ears or ear shanks. Overwintering Crambidae) is a well- larvae are light yellow-white and do not pupate until known caterpillar pest the following spring. Only faded spots are present on of corn. Its biology is overwintering larvae. similar to European corn borer. The moths Hosts, Life History and Distribution: Southwestern are dull white or buff- corn borer has relatively few hosts. Corn is the colored and about 1 primary host, but larvae are occasionally found on inch long, although sorghum and Johnsongrass. The SWCB is primarily their size can vary. distributed in the southern United States and Mexico. Southwestern corn borers (SWCB) lay flattened eggs Cold winter temperatures in most of the Midwestern in an overlapping mass reminiscent of fish scales. Corn Belt limit the northern range of this insect. Egg masses typically range from 2-6 eggs (whereas European corn borer egg masses normally have 8-40 A female moth only lives 5-7 days but may lay 250 eggs). Eggs are white when initially laid. They then eggs during her life span. Eggs take about five days develop red stripes within about 36 hours. Eggs that to hatch. Except for overwintering larvae, it takes are totally black have been attacked by a tiny parasitic about 20 days for a larva to develop into a pupa. wasp. Moths from the overwintering generation mostly emerge in May and produce the first generation of Larvae have brown head capsules. Small larva are borers. In Tennessee, the next moth flight and a second generation of borers typically occurs sometime in Reduced tillage systems favor SWCB, because larvae mid July. A third moth flight and a third generation of overwinter at the base of stalks. However, tillage larvae occurs during August and September. will have little impact on potential infestations the following year unless it is done across a relatively Pest Status and Injury: Southwestern corn borer is large area. Moths can re-infest an area from an important pest of corn. This species is generally neighboring, untilled fields. Some kinds of Bt corn the most common “borer” in the western part of (e.g., YieldGard® and Herculex®) produce a toxin that Tennessee. On whorl-stage corn, hatching larvae is very effective in controlling corn borers. As part of move into the whorl and feed on leaves. Feeding signs an insecticide resistance management plan, a refuge of include elongate window-pane lesions on emerging non-Bt corn is required for Bt corn. In cotton-growing leaves. In tasseling corn, small larvae usually feed areas of Tennessee, only 50 percent of a grower’s corn behind leaf collars and between ear husks. Most larvae acreage can be planted with corn that has a single Bt will be found within two leaves above or below the ear trait for controlling corn borers. Up to 80 percent Bt leaf. Older larvae tunnel into the stalk, in ear shanks, corn can be planted in non-cotton areas. Corn varieties or feed on ears until they pupate (usually inside having two or more Bt traits (i.e., YieldGard VT the stalk.) Tunneling interferes with nutrient and Pro®, SmartStax®) are being developed. A smaller water flow within the plant and to the ears. Tunneled refuge of non-Bt corn will be required for these newer shanks may break, causing ears to fall on the ground. technologies, and these technologies also provide Most second-generation larvae will overwinter. improved control of corn earworm and fall armyworm. Overwintering larvae usually girdle the stalk from the inside. Girdles are normally located 1-6 inches above It is recommended that non-Bt corn be treated with the ground and are capped with frass and plant debris. insecticides if 20-30 percent or more of plants are Girdling often results in lodging, particularly in high infested with eggs or small larvae. Insecticide choices winds or when infested corn is not harvested in a for control of SWCB are listed in the Tennessee Insect timely manner. Lodging can dramatically reduce yield Control Recommendations for Field Crops (PB 1768). and slow harvesting operations. It is important to make insecticide applications before most larvae begin tunneling into the stalk; otherwise, Management Considerations: SWCB population poor control will result. In whorl-stage corn, high levels vary widely from year to year and across volumes sprayed directly into the whorl will provide different locations. Both first- and second-generation the best results. Aerial applications are typically larvae may cause economic damage to corn. A partial needed in tasseling corn. Pheromone moth traps are third generation is too late to affect fields planted very useful in determining the timing and relative during the recommended planting window. SWCB size of moth flights. However, they do not necessarily populations are lowest during the first generation, so correlate with subsequent larval populations in widespread infestations are less likely at this time. individual fields. These traps should be used to help Moths often concentrate their oviposition in a few time scouting efforts to when infestations are likely fields, especially targeting early-planted fields. The to be present. There is usually a delay of 7-14 days second generation affects more fields, and unlike between observing an increase in moth catches and a the first generation, populations are typically highest corresponding increase in egg or larval populations in in late-planted fields. However, depending on the nearby fields. timing of moth flights, many fields can potentially be infested. Planting early in the recommended planting References: window is suggested to avoid late-season infestations Handbook of Corn Insects, K. L. Steffey et al. (eds.), of SWCB and other caterpillar pests (e.g., European Entomological Society of America, 1999. corn borer, fall armyworm and corn earworm). Controlling the Southwestern Corn Borer. C. Patrick, S. Stewart and A. Thompson, University of Tennessee Extension, SP503-E.

2 Eggs of southwestern corn borer Small larva

Southwestern corn borer moth Larger larva in stalk

Leaf feeding damage

Girdled stalk Overwintering larva

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